Method for the quality control of material layers

ABSTRACT

Disclosed is a method for quality control of a material layer, which involves providing the material of the layer with an agent for absorbing an electromagnetic radiation, irradiating the surface of the layer with an electromagnetic radiation, and measuring the amount of light emitted by the material layer, for example, reflected radiation or fluorescence radiation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for the quality control ofmaterial layers. Methods of this type are used in particular in qualitycontrol and quality assurance of articles which can be mass-produced.

2. Description of the Related Art

According to WO 98/18135, the quality of a coating is checked by lightbeing radiated onto the coating and the transmitted light beingdetected. From the transmission or respectively the absorption of thelight as it passes through the coating, conclusions are made about localor overall defects in the coating.

U.S. Pat. No. 4,302,108 discloses a method in which a coating is alsoscanned with a light beam. What is crucial here is that the angle ofradiation onto the coating is so selected that at least a portion of thebeam is reflected. The intensity of the reflected beam is then detectedin order to determine surface anomalies of the structure. By means ofthis method, which is substantially based on an angle of radiation whichleads to a reflection at the surface of the coating, surface defects ofthe coating can be detected. These described methods relate to planarsurfaces.

BRIEF SUMMARY OF THE INVENTION

Proceeding from this prior art, the object of the present invention isto make available a method for the quality control of material layers,by means of which the quality of the layer can be measured in anon-destructive manner in respect of layer thickness and surfacedefects, simply, quickly and cost-effectively.

This object is accomplished by the method according to the preamble ofclaim 1 in conjunction with its characterising features. Furthermore,fields of use of the method are quoted in claims 11 and 12.

BRIEF DESCRIPTION OF THE DRAWINGS

Some examples of the method according to the invention are given below.The figures show:

FIG. 1 a measuring arrangement for a method according to the invention;

FIG. 2 the method according to the invention;

FIG. 3 the correlation between colorant concentration and signalintensity;

FIG. 4 the correlation between membrane layer thickness and intensity ofthe signal for the colouring agent scarlet red;

FIG. 5 the relationship between membrane layer thickness and intensityof the signal for the fluorescent colouring agent curcumin, and

FIG. 6 signals from the measurement of two ion-selective electrodes.

DETAILED DESCRIPTION OF THE INVENTION

Advantageous developments of the method according to the invention aregiven by the dependent claims.

The invention proceeds from a method according to U.S. Pat. No.4,302,108, in which a specific angle of irradiation has to be observedin order to generate reflected radiation. Differing from this, however,the present invention proceeds from the material of the layer to beexamined being provided with an agent which absorbs electromagneticradiation (absorber), for example a colouring or fluorescent agent, andthe light then reflected from the material or emitted as fluorescentlight is measured. Here the light, which after penetration into thelayer is reflected in deeper planes of the layer and/or fluoresced andfor example leaves the layer again via the surface, is also detected.The present invention exploits first of all the fact that thereflectivity is dependent on the absorption capacity of the respectivematerial and therefore the luminance factor of the material depends onits absorption coefficient. In this way it becomes possible also tomeasure layer materials which for example for their part do not have anysubstantial inherent absorption in the ultraviolet or visible lightranges. Nevertheless, by colouring the material with an agent whichabsorbs electromagnetic radiation, absorption can be created and thusthe luminance factor can be increased independently of the angle ofirradiation.

As agents which absorb electromagnetic radiation can be consideredcolorants, e.g. colouring agents such as dyes or pigments, or alsoagents which absorb electromagnetic radiation such as UV absorbers orfluorophores.

In the case of the use of a fluorophore, it is not the radiation emittedby scattering from the surface but the fluorescent light of the layeremitted over the surface which is determined. Both processes, thedetermination of the back-scattered portion or of the fluorescent light,are equally suitable for the present method.

Through the method according to the invention, a generally usable, quickand cost-efficient on-line quality control is possible, for example forsurfaces which can be mass-produced. This method is non-destructive andnon-contact and can be used to determine layer thicknesses or alsosurface defects or irregularities. Thus consequently amounts of filling,layer thickness distributions, defects such as holes, accumulation orinhomogeneities can be detected in any materials. The method isparticularly advantageously suitable for the quality control and qualityassurance of plastics membranes, polymer membranes, in particularion-selective membranes or hydrogels or the like.

Such plastics membranes are used for example in ion-selective electrodesfor biosensors.

With the method according to the invention, not only membranes but alsohydrogels can be examined. If the colouring agent necessary for thedensitometric measurement is not to be dissolved out of the membrane inthis process, it must be bound to particles. These particles are in turnenclosed by the hydrogel. Latex particles are particularly suitable assuch particles.

The layer can advantageously be scanned point-by-point, it beingpossible for the scanning to take place in a specific direction. By thismeans it is possible, for example, to measure in sequence a lineararrangement of ion-selective electrodes in respect of their membranelayer thickness. In particular the incorporation of the method forquality control according to the present invention in a sequentialproduction cycle is possible.

Since the luminance factor or respectively the fluorescence of amaterial depends on its absorption coefficient, the luminance factor orthe fluorescence is also bound to the wavelength of the incident light.It is advantageous, therefore, if the electromagnetic radiation radiatedonto the layer has a wavelength which corresponds to an absorption bandof the absorber. However it should be borne in mind here that, via thechoice of the irradiated wavelength or the choice of the colorantconcentration in the membrane, a measurement is taken in a region inwhich a clearly evaluable relationship, ideally a linear relationship,arises between the amount of reflected light or respectively fluorescentlight and the layer thickness of the membrane.

Particularly suitable for the present method are the colouring agentscurcumin (1,7-bis(4-hydroxy-3-methoxyphenyl) 1,6-heptadiene-3,5-dione),scarlet red (1(-2-methyl-4-o-tolylazo-phenylazo)-2(naphthol), or Sicodopblue (pigment paste, trade name of an insoluble colorant produced by thecompany BASF, chemical structure not made public). Curcumin is afluorescent colorant which has the additional advantage of beingnon-toxic as a food colorant.

FIG. 1 shows an arrangement for measuring according to the method of theinvention. This arrangement has a computer 1 and a TLC scanner 2(thin-layer chromatography scanner) as a densitometer. The TLC scanner 2and the computer 1 are connected via a connecting cable 3, via which thecomputer controls the densitometer 2 and records the measurement curves.

For measuring, the object to be measured is positioned on the bed of thescanner 2, fixed with magnetic strips and inserted into the densitometer2. For measurements in the visible light range, a tungsten lamp is usedas the light source, whilst a mercury lamp is selected to stimulatefluorescence. In the latter case, in addition a corresponding cut-offfilter must be positioned before the measuring photomultiplier of thedensitometer 2, in order to separate spectrally excitation light andfluorescent light.

After input of the corresponding measuring parameters into the computer1, the measurement begins. The light beam travels in one direction overthe sample, as represented in FIG. 2. The intensity of the reflectedlight is measured and is represented as absorption on the basis of itsproportionality to the absorption. Alternatively the fluorescent lightis measured.

FIG. 2 shows the measurement of a sequence of ion-selective electrodes.

To produce ion-selective electrodes of this type, plastics membraneswith intercalary analyte-specific ionophores are used. In the case ofsensors which are produced in double-matrix membrane technology, a PVCmembrane is, for example, applied in liquid form to a porous material. Aportion of the membrane penetrates into the matrix, the remainder isuniformly distributed over the porous layer in a cavity. In thisprocess, the electrode is sealed against the measuring medium. In orderto minimise leaks, according to the present state of knowledge, an atleast four-fold filling of the electrode with PVC cocktail is necessary.Maintaining the quality requirement of uniformity of the filling andimpermeability is then checked with the densitometric method accordingto the invention. To this end, a corresponding colouring or fluorescentagent, for example, which absorbs electromagnetic radiation, is added tothe PVC membrane in the liquid state, before it is poured. As absorbentsubstances can be considered the above-mentioned pigments or colorants.Furthermore, fluorophores can be used.

FIG. 2 now shows a sequence of ion-selective electrodes 6, 6′, 6″, 6″′,6″″, mass-produced in this way, which are disposed on a measuring table4. The electrodes are produced as individual sensor strips in a sensorarray 5 and are disposed the one beside the other. The electrodes 6 to6″″ each have a measuring head 7. This measuring head 7 contains theion-selective membrane to be measured.

Excitation light 8 is now radiated onto the measuring table and thelight 9 reflected from the measuring heads 7 is then measured andrepresented as absorption. Via the amount of reflected light, the layerthickness and also the homogeneity of the ion-selective membrane in themeasuring heads 7 is thus scanned.

The measuring table 4 is displaced in the direction of the arrow, suchthat on the one hand the individual sensor strips are measured in thesequence 6″″, 6″′, 6″, 6′ and finally sensor strip 6, and on the otherhand, the profile of the ion-selective membrane of each individualmeasuring head 7 is also determined in the direction of the arrow.

In order to adjust the measuring range of the densitometer, beforemeasurement a null balance is carried out with an uncoloured matrix.

FIG. 3 shows a calibration curve for electrode membranes of the typedescribed, with different concentrations of the colorant scarlet red. Upto a concentration of approximately 0.12 g/l scarlet red in themembrane, can be recognised an extra linear interdependency of intensityand amount of colorant in the membrane. In order, within the frameworkof quality assurance, to be able to distinguish well between differentmembrane thicknesses, the attained amount of colorant should, with asufficient filling of the membrane cavity of the measuring heads 7, liebelow this concentration at least by a factor of 2. In the case ofhigher concentrations, one leaves the linear measuring range and theresolution of the measurement is poorer, if not impossible.

FIG. 4 shows a measuring curve which has been determined according to amethod as shown in FIG. 2, on five consecutive measuring heads 7. Theindividual measuring heads have here been filled once (reference numeral1) twice (reference numeral 2), three times, four times or five times(reference numerals 3, 4 or 5 respectively) with an ion-selectivemembrane, which contained 0.0224 g/l scarlet red. On the basis of thefixed cavity dimensions for the ion-selective membrane, measuring headswith five different layer thicknesses of the ion-selective membrane wereproduced in this way.

As can be immediately recognised, with the same colorant concentrationin the ion-selective membrane, the layer thickness results in fivedifferent signal intensities, the signal intensity correlating directlywith the layer thickness. This provides the proof that the methodaccording to the invention can be used to determine layer thicknesses inion-selective membranes.

FIG. 5 shows a further measuring curve which has been determinedaccording to a method as shown in FIG. 2, on five consecutive measuringheads 7. The individual measuring heads were here filled once to fivetimes (reference numerals 1 to 5) with an ion-selective membrane, whichcontained 0.0224 g/l curcumin. In this way, five measuring heads wereproduced, the membrane layer thicknesses of which vary between thesingle original layer thickness and five times the original layerthickness in measuring head 1. As can be immediately recognised, heretoo, with the same colorant concentration in the ion-selective membrane,the layer thickness results in five different signal intensities for themeasured fluorescent radiation, the signal intensity for measuring heads1 to 4 correlating with the layer thickness. The signal intensitybetween measuring head 4 and measuring head 5 no longer differssubstantially, since here the saturation region of the signal has beenreached as a result of the large layer thickness. The colorant curcumincan consequently be used in the selected concentration of 0.0224 g/l inorder to distinguish the layer thicknesses of measuring heads 1 to 4from one another. In order also to correspondingly detect larger layerthicknesses, such as those of measuring head 5 for example, a reductionof the concentration of the fluorescent colorant curcumin in themembrane solution would be necessary. This provides the proof that themethod according to the invention can also be used via the detection offluorescent light to determine layer thicknesses in ion-selectivemembranes.

FIG. 6 shows a sequence of two measuring heads 7, 7′, which are pushedthrough in the direction of the arrow with the measuring table 4represented in FIG. 2, below an excitation light beam. The associatedsignals of the reflected light are represented in the lower portion ofFIG. 6.

The measuring heads 7, 7′ each have one silver electrode 10 or 10′respectively, which surrounds a cavity in an annular manner. Into thiscavity is filled a membrane filling of an ion-selective membranematerial 11, 11′. The ion-selective membrane material 11, 11′ iscontacted on the rear side, not shown here, of the measuring head withthe silver electrode 10, 10′ for derivation of the signals.

The membrane filling 11 or 11′, while still in the liquid state beforebeing filled into the cavity, was coloured with scarlet red in identicalcolorant concentration for both measuring heads 7, 7′.

As is clear in FIG. 6, not only do the coloured surfaces, coloured withscarlet red, of the membrane fillings 11, 11′ absorb and reflect but sodo the silver electrodes 10, 10′. This must be taken into account in thesubsequent digital evaluation of the measured signals.

Nevertheless it can be clearly recognised that the membrane fillings 11,11′ generate a signal which has a plateau and which corresponds to thecolorant concentration or the layer thickness of the membrane fillings11, 11′. Furthermore it can be recognised that the plateaux of thesignals of the membrane fillings 11, 11′ have a ridge in which theintensity is slightly lower than at the edges of the plateaux. This canbe traced back to the fact that, after filling, the membrane cocktailrises up the edge of the cavity. Thus in the middle of the membrane asurface is produced with less membrane cocktail, i.e. reduced layerthickness of the membrane fillings 11, 11′. This also proves that themethod according to the invention is suitable for detecting layerthickness profiles and defects within the membrane and also on themembrane surface.

1. A method for detection of layer thickness or respective amount offilling, layer thickness distribution, defect, accumulation orinhomogeneity within a material layer, said material layer having asurface, comprising irradiating the surface layer with anelectromagnetic radiation, and determining the amount of light emittedfrom the material layer, wherein the material of the layer is providedwith an agent which absorbs the radiation, the material of the layerbeing mixed with the agent which absorbs the radiation before the layeris created.
 2. The method according to claim 1, wherein the material ofthe layer is provided with the colouring agent as the agent whichabsorbs the radiation.
 3. The method according to claim 2, wherein thecolouring agent is a colorant or a pigment and/or with a UV absorberand/or with a fluorophore.
 4. The method according to claim 1, whereinthe layer is scanned with the electromagnetic radiation.
 5. The methodaccording to claim 4, wherein the layer is scanned point-by-point. 6.The method according to claim 4, wherein the layer is scanned in alinear manner with the electromagnetic radiation.
 7. The methodaccording to claim 1, wherein the electromagnetic radiation radiatedonto the surface of the layer has a wavelength which corresponds to anabsorption band of the agent which absorbs the radiation.
 8. The methodaccording to claim 1, wherein the electromagnetic radiation isultraviolet and/or visible light.
 9. The method according to claim 1,wherein the agent which absorbs the radiation is added to the materialof the layer bonded to particles.
 10. The method according to claim 1,wherein the agent which absorbs the radiation is added to the materialof the layer bonded to latex particles.
 11. The method according toclaim 1, wherein curcumin, scarlet red and/or Sicodop blue are added tothe material of the layer as the agents which absorb the radiation. 12.The method according to claim 1, wherein the material layer is a plasticmembrane, a polymer membrane, an ion-selective membrane, or a hydrogel.13. The method according to claim 1, wherein the material layer is anion-selective membrane.
 14. The method according to claim 1, wherein thelight emitted is reflected radiation and/or fluorescent radiation.